Tag: AM Transmitters

It is a power supply from a Broadcast Electronics AM5E transmitter. Here is another view:

Broadcast Electronics AM5E power supply mating connector

As you can see, there was a small fire started in the mating connector for the transmitter wiring harness. I did not install this unit so I have no way to know for sure what happened, but I suspect that the mating connector was not pushed all the way in during installation. In this business, really in all engineering fields, it is the little details that will catch up with you.

I know that one of the stations I used to work at had a fire at their electrical service panel at the FM transmitter site, after they installed a new transmitter. This happened after I departed for greener pastures. In any case, it is very important to torque the connections on any service disconnect or circuit breaker to the panel manufacture’s specifications. I also check the lugs every so often with a Fluke 62 mini IR temperature meter. Any loose connections will show up as hot spots, which can be fixed before the fire breaks out.

All current carrying electrical connections should be double checked for solid connections before the transmitter is turned on, then check periodically thereafter for heat buildup and or heat damage.

I began fooling around with radios when I was 10 years old or so. First, I built one of those shortwave radio kits from Radio Shack, which was back when they still sold radios.

Then I bought a small tube type AM transmitter at a garage sale. The woman there said her son built it several years ago from a kit and it had the instruction manual. I don’t even know who made the kit. After some experimentation and changing out some tubes, I got the thing to transmit on about 1600 kHz, although it was a little hard to nail down as it drifted quite a bit until everything heated up. I don’t know what power that thing put out, but it was certainly less than a watt.

All of this lead to a brief stint in the military as a radioman. That was an interesting field, albeit different from what I thought it would be when I signed up. It was during this time that I did some part time work at an AM/FM/TV station assisting the Chief Engineer. Once it was established that I actually knew something, my responsibilities grew until I was assigned the AM/FM part of the deal.

After a year of that, I moved to a different city for family reasons and took the Chief Engineer job at a local AM/FM station. The AM station was a 50,000 watt directional in the high end of the band which had a Harris MW-50B transmitter. My previous station had a Bauer 10,000 D AM transmitter. What could be so different? Plenty I learned, on my second day.

Harris MW50B transmitter with 50 KW air cooled power supply

We were subjected to a wicked lightning storm, which, Murphy being present, took out the main transmitter. The backup was a GE BTA25 which was running at half power because of the age of the 5891 final tubes.

The symptoms of the MW-50 where as follows: It would run along fine then there would be a big blue flash and a cannon shot boom, followed by the step start relays cycling and it would come back on the air. There were no overload lights nor any other symptoms leading up to the overload or subsequent to it.

I began by killing the power and shorting out all the high voltage parts with a shorting stick. I noticed that things inside this transmitter where a little unusual, so I got the manual out and started reading. The most unusual aspect of this transmitter is the 25 KV isolated box that the PA stage occupies. 25,000 volts DC is a great big potential and what I found over the years is that this transmitter needs to be kept very clean. Of course, this unit had not been, and that was a part of the problem.

The other unique aspect of this transmitter is the damper diode, which is required by PDM transmitters to conduct voltage during the negative modulation peaks. If the damper diode breaks down for any reason, the PA supply voltage tries to go to infinity, which is a good deal larger than 25KV and all sorts of problems begin.

To make a long story somewhat shorter, this is the problem I had. The solid state damper diode had one bad section, which was causing all sorts of corona problems during heavy negative modulation peaks. It took a call the Harris factory to determine this. The entire diode assembly needed to be replaced because every section is matched. That cost a couple of thousand dollars as I recall.

While I was working on the MW-50B transmitter, I was not impressed. It seemed a little cheap and flimsy. Later, when I voiced my concerns with the station management, the Harris transmitter salesman stopped by and said I needed to get with the program if I wanted to work in that market. This was a Harris town you see, if you start bad mouthing our products, you’ll be the one to suffer. Well, he retired, I kept looking around for other AM transmitters. Three years later I went to work for the competitor across town. Today that station has a Nautel ND-50.

The MW50 went off the air once every 6 months for the entire time I worked at this station. It was always something different, power supply rectifier, bad PDM board, bad directional coupler, arcing insulator on the isolated box, etc. I began to feel it didn’t like me, and I know I didn’t like it. In fact, you could say I have never really liked Harris transmitter products ever since.

Update: Okay, I left a few things out of the narrative:

The 50 KW air cooled power supply was the light weight version. Most MW-50 transmitters had 100 KW oil cooled supplies. The problem with the 50 KW power supply was it was designed with a zero safety factor. All of the rectifier were running at or near maximum current and voltage. It only took one of 144 diodes to go bad, either short or open, and the whole transmitter would crash. Again, no overload lights or other indications of problems. We later installed air flushing fans in the power supply cabinet to keep things cool and that helped out quite a bit.

The other thing was a DC feedback sample to the PDM card. It seems that if the filaments were turned off before the bleeder resistors took the 25 KV supply to zero, the remaining voltage would be routed to the PDM card via the DC feedback sample, blowing the foil off of the circuit card. We fixed this by installing a gas discharge tube with a series resistor at the connection point for the DC feedback sample.

Then there are the infamous 1N914 diodes in the directional coupler that Dave points out below.

This is one of my favorite old transmitter memories. Back when I landed my first Chief job, I was working for an AM/FM combo. The AM station was a 50,000 watt flame thrower that first went on the air in 1947. The original transmitter, a General Electric BT-25-A was still in service as a backup unit. These pictures are from the last night it operated, December 16, 1993. The bank made us remove all of the PCB transformers and capacitors before they would refinance their loan. Of course that was most of the transmitter, the rest of it was scraped or sold for parts.

This is a long transmitter, GE BT-25-A looking from the control cabinet

The transmitter takes up the entire span of the room. There were eight large cabinets, each with its own stage or section. The stages were connected to each other by a wiring trough in the floor. The transmitter used lead jacketed cable within and between sections.

IPA stage with "multi-meters"

The IPA section had been modified to use 833A tubes. This is where the RF was developed and amplified for the final section. It is in the middle cabinets of the transmitter, the audio and control section being to the left, the PA and PA power supply being to the right.

PA section. Those are WL 5891 tubes.

Final section. There were three tubes, only two where in use at any given time. The third tube was a spare which could be quickly placed in service by throwing a knife switch and moving those bars on the back wall around. This picture was taken with filament voltage only, we had to close the door to turn on the PA voltage.

air cooling blower

The transmitter was cooled by this blower which faces down into the floor. A concrete duct work carried the air to the various stages of the transmitter. The blower is powered by a 2 1/2 HP motor. There were two blowers, one in use and one is standby. Behind this is an air mixing room and filter room. During the winter time, the transmitter waste heat was used to heat the building by closing a series of ducts and opening other ones.

Modulation transformer and modulation reactor

This is in the transformer vault. The unit to the left is a modulation transformer, it was 7 feet tall. Directly in front is the modulation reactor and just out of the picture to the right is the plate transformers. The plate supply was 480 volts 3 phase. The other piece of green equipment is a hydraulic tube jack, to get the 5891 final and PA tubes out of their sockets.

The transformers where what contained most of the PCBs. The modulation transformer contained about 150 gallons of Pyranol, the GE trade name for their transformer oil. Pyranol contained greater than 750,000 parts per million PCB.

It is a shame we had to kill this transmitter, is sounded wonderful on the air. The day we signed it off, there was nothing like it, not the Mw-50B that replaced it, nor the Nautel ND-50 that replaced the MW-50, nor the DX-50 at the competing station across town.

Engineers are funny. We all have our likes and dislikes and our reasons for both. I don’t really like Harris products. Even when I was in the military, their stuff seemed a little “light.” I suppose having to deal with an MW-50B transmitter at my first full time chief engineer gig didn’t help that impression. The MW-50 would “blow up” every six months or so. I say blow up because that is the only way I can describe it, no overload lights or any other indication of trouble until the blue lightning flashes and thunder from the PA section. What a POS.

Other Harris transmitters, such as the SUX-1, FM20H, Gates-1 etc have also left me less than impressed.

In order of preference, my choice of AM transmitters would be:

Any Nautel solid state unit. Nautel makes good equipment that is well supported.

Any BE solid state transmitters. I favor the A model over the E model, but both are good. One condition, they must absolutely be well grounded and all of the toroid filters provided by the manufacture must be used when installing.

Any tube type Continental transmitter. There are older units, I believe 816R but they work well and sound good on the air.